In prior art high pressure furnaces, uniform temperature distribution was sought either by circulating the pressurized atmosphere through the furnace or by carefully arranging the heating elements and individually controlling them. This application is directed to improvements in the gas circulation technique wherein no mechanical means are used to promote circulation.
Prior art methods of promoting gas circulation without mechanical apparatus such as fans are taught in U.S. Pat. Nos. 3,419,935 to Pfeiler et al., 3,548,062 to Smith and 3,571,850 to Pohto, for example. In U.S. Pat. No. 3,548,062 to Smith, a technique is disclosed in the specification for promoting gas circulation comprising a baffle between the workpiece and the heating elements positioned more or less uniformly along the inside wall such that gases are drawn up through the space between the baffle and the heating elements and are permitted to flow down over the workpiece being cooled by transferring heat to the workpiece. As the workpiece heats up, the hot gases passing the workpiece are not as rapidly cooled and therefore the circulation of the gases slows down. In U.S. Pat. Nos. 3,419,935 to Pfeiler et al. and in 3,571,850 to Pohto, gas circulation is promoted by a liner between the workpiece and a furnace wall and by heating gas just below the workpiece and allowing the gas to rise transferring heat to the workpiece as it flows past and then permitting the gas to flow back down between the liner and the furnace wall. These furnaces have been limited to the use of metal resistance elements due to the configuration of the hearth and furnace bottom. This is a substantial disadvantage as graphite and silicon carbide heating elements offer distinct advantages in certain environments.
There currently exist numerous uses for apparatus that treat a specimen or workpiece at high pressures and high temperatures including, for example, gas pressure bonding furnaces and hot isostatic pressing apparatus. In these apparatus, it is typical to treat a workpiece at 1000.degree. C. and 15,000 psi although these are not the maximum temperature and pressure conditions encountered. Suitable apparatus for these applications generally comprises a furnace within a pressure vessel or autoclave. The furnace provides the heat to the workpiece and protects the vessel from excessive temperature. The vessel maintains the furnace and the workpiece at the desired pressures.
For a given pressure, the diameter of the pressure vessel determines the minimum safe thickness of the vessel wall. To avoid extremely heavy vessels, it is desirable to reduce the vessel diameter as much as possible. Stated another way, the space between the interior of the vessel lining and the workpiece should be very small even though this is the space occupied by the furnace.
In most processes, it is essential that the temperature of the workpiece be extremely uniform. Otherwise, problems may result from differential thermal expansion of the workpiece. Thus, the furnace portion of the high pressure-high temperature apparatus must distribute the heat evenly to the workpiece.
It is an advantage of this invention to provide an autoclave or pressure vessel-furnace structure that minimizes the diameter of the pressure vessel, while at the same time providing for even distribution of heat to the workpiece in a way to obtain uniform workpiece temperature.